I believe 'magic' would be better suited for an extra sense than 'Pinkie'. Earth ponies get passive magic, as opposed to unicorn's active magic. There's a fanon theory that pegasi have a passive magic ability that allows them to basically jellify and contact air, and effectively 'swim' through it. Look at Cloudsdale.

Earth pony's passive magic is how AJ's extremely strong, and why earth ponies tend to be able to get high positions in society (+charisma). It could also explain Pinkie's ludicrous speed, and her other traits, such as her extra senses.

Or it could be a cartoon and we're all overanalyzing.

correction their skin is their largest organ